35meter Deep Space Antenna

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• 35-meter Deep Space Antenna
• Pointing Calibration System

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Authors

• Ron Osterried
• SED Systems, a division of Calian Ltd.,
  Saskatoon, Saskatchewan, S7N 3R1, Canada
• Peter Droll
• European Space Operations Centre of ESA (ESOC),
  Darmstadt, 64293, Germany
• Bob Plemel
• SED Systems, a division of Calian Ltd.,
  Saskatoon, Saskatchewan, S7N 3R1, Canada

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Background

• In 2005 SED Systems installed and commissioned
  the X/Ka-band 35-meter antenna system at
  Cebreros, Spain including a pointing calibration
  system (PCS)
• The pointing error (PE)requirement is 5.5 mdeg
  or 1.2 dB under worst conditions
• The PCS is fully automated and under remote
  control.
• The PCS measures and provides corrections for
  systematic pointing error.

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Systematic Pointing Error Sources

• Sources of pointing error
• Refraction (actively corrected)
• Gravity (SPEM)
• Misalignment (SPEM)
• Beam squint (SPEM)
• Thermal deformation (active thermal correction)
• Wind (stiff antenna structure and drive)

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Systematic Pointing Error Model
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PCS System Design Main Elements

• The main elements of the PCS are
• Pointing Calibration Computer (PCC) Server
• PCS Workstation (PWS)
• Radiometer/Noise Diodes
• Temperature Measurement System (TMS)
• The main interfaces of the PCS are
• Antenna Control Unit (ACU)
• Front End Controller (FEC)
• WSDS (Weather Station Data Server)
• Noise injection RF Couplers before LNAs
• Downconverter IF outputs to radiometer

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PCS System Design Block Diagram
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PCS System Design PCC and PWS

• Pointing Calibration Computer Server (PCC)
• Runs the PCS application software to control the
  pointing calibration process
• IRIG-B time interface
• LAN interfaces to FEC, ACU, WSDS (weather
  server), radiometer, and TMS
• Pointing Calibration Workstation (PWS)
• Provides the local user with a graphical
  interface for the PCS
• A remote access capability is provided to allow
  the same functions from a remote workstation
• MC, initiate measurements, view historical
  results, interactive SPEM coefficient calculation

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PCS System Design Radiometer

• Radiometer
• Used to measure system noise temperature
• Standalone chassis / LAN interface
• Separate inputs for each downlink frequency band
  and polarization
• 100 MHz bandwidth at X band, 200 MHz at Ka band
• IRIG-B time interface
• Scheduled measurements to 1 msec timing accuracy
• Noise adding and total power modes
• Custom designed circuit boards populated and wave
  soldered in plant at SED
• Menu driven, intuitive local front panel control

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PCS System Design Radiometer Boards
Radiometer MC Processor Board
Radiometer RF and Measurement Board
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Temperature Measurement System

• Antenna Physical Temperature Measurement System
  (TMS)
• 252 temperature sensors located on the main
  reflector back-structure, K-strut, ballast
  cantilevers and the subreflector quadrapod struts
• Temperature data is used by the PCS to calculate
  thepointing error due to thermal distortion of
  the mechanical structure
• Sensitivity of pointing change for a unit
  temperature change for each sensor node is from
  finite element analysis
• Update rate 30 seconds

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PCS TMS Window
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Installed PCS Antenna Rack
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PCS Operating Modes

• PCS Operating Modes
• Calibration Mode and Schedule
• PE Measurement (within a calibration)
• SPEM Calculation and Transfer to ACU
• Compensation
• Operator Direct PE and Noise Temp Measurements

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Calibration Mode

• Calibration Mode
• PCS enters calibration mode if PCS has control
  and a calibration is requested locally or
  remotely
• Typical calibration for one frequency band, one
  polarization, will take approximately 8 hours
• Approximately 230 PE measurements are taken
• Main elements of the calibration process
• Scheduler
• Individual PE measurements
• SPEM coefficient calculation using results
• Coefficient Transfer to ACU

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Scheduler

• The operator begins by specifying
• Start time and duration of a calibration
• Constraints, if any (min/max elevation, min flux)
• The scheduler automatically selects calibration
  sources defined in the PCS database
• Sources have an angular extent of less than 1
  mdeg
• Flux density gt 1.5 Jy in both X- and Ka-band

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Scheduler (contd)

• The scheduler automatically builds a measurement
  schedule
• Maximizes the number of PE measurements to be
  made in the calibration period by minimizing
  antenna motion
• Provides near-uniform distribution over the
  hemisphere
• Gives preference to radio stars crossing through
  infrequently covered areas of the sky
• Gives preference to higher flux density sources
• Avoids sun and moon

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PCS Scheduler Output Window
Scheduler Output for 8 hour Calibration (227 PE
Measurements)
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PE Measurement Method

• PE Measurement Method
• PCS calculates the track of the star using
  TpointTM slalib Software
• Uses a grid of Az and El offsets around the
  nominal position of the star
• X-band beamwidth 64 mdeg
• Ka-band beamwidth 17 mdeg

• PCS commands the antenna to follow a trajectory
  through the grid points
• PCC schedules N x N radiometer measurements of
  system noise temperature centered at each grid
  point

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PE Measurement Method (contd)

• After the scan is complete, radiometer results
  are read
• Typical time for one PE measurement is 1 minute
  including calculations
• A mathematical model is fit to the measured Tsys
  data to determine the location of the RF beam
  relative to the commanded position
• PE measurement data is stored in the database
  including raw pointing error which is the
  measured pointing error plus all current
  systematic corrections
• PE measurement results can be viewed live or
  historically

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PE Measurement PCS Result Window
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SPEM Calculation

• SPEM Calculation
• Can use between 30 to 10,000 PE measurements
• Mathematical model (SPEM) is fit to the PE
  measurements
• Model for DSA2 contains 10 independent
  coefficients (14 total with redundancies)
• Interactive calculation tool with exclusion to
  user specified statistical threshold (e.g. 3
  sigma)
• Operator defined database filters. Filters are
  logged with all calculation results.
• Residual pointing error graphical views provided
  by PWS gui

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SPEM Calculation Raw Pointing Error
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SPEM Calculation Residual Error
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Transfer and Compensation Mode

• SPEM Transfer
• After SPEM Calculation, the PCC transfers SPEM
  coefficients to the ACU
• ACU is in maintenance mode during transfer
• Compensation Mode
• Normal operating mode of the PCS
• PCS calculate thermal correction values from the
  current TMS data and transfers the values to the
  ACU
• ACU applies corrections to all commanded antenna
  positions
• SPEM, Refraction, Measured Tilt, thermal from PCS

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Other PCS Capabilities

• Direct Mode Noise Temperature Measurements
• Measure Tsys at any azimuth/elevation
• Useful for monitoring system performance over
  time especially for changes due to the cryogenic
  LNAs
• Measure elevation dependence of the noise due to
  atmospheric thermal emission
• Direct Mode PE Measurement
• Verify pointing by selecting any visible radio
  star
• Can be used to repetitively measure PE on a
  single radio star
• Can be used for G/T measurements using offline
  formulae
• Available under remote control

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Direct Mode PE
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Beam Squint
Measured Difference between LHCP/RHCP SPEM (1-2
mdeg)
Predicted Difference between LHCP/RHCP (1.8-1.9
mdeg)
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Moon Scan
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Conclusion

• PCS accurately determines systematic pointing
  error
• Ideal for high usage TTC systems full
  automation allows effective use of small blocks
  of antenna time
• PCS provides flexible tools for monitoring and
  antenna performance

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References

• Droll, P., Cozzani, A., Sieber, R., Schäfer, M.,
  A Thermal Distortion Model for the ESA DSA in
  Perth for Compensating Thermally Induced Pointing
  Errors, Second ESA Workshop on Tracking,
  Telemetry and Command Systems for Space
  Applications 2001, ESA/ESTEC.
• SLALIB, Positional Astronomy, Software Package,
  Tpoint Software, Abingdon UK, 2003.
• Vertex Antennentechnik, Technical Note Systematic
  Pointing Error Model, VA Document No.
  TN1003057-31440, Release 1.5, Internal Document

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